1. Field of the Invention
The present invention relates to an acoustic transducer and an underwater sounding apparatus for detecting underwater objects, such as fish schools, by use of ultrasonic waves.
2. Description of the Related Art
Scanning sonars are conventionally known acoustic apparatuses for obtaining information on underwater objects, such as fish schools, distributed in a wide underwater area. A scanning sonar is provided with a transducer for transmitting an acoustic sounding beam in specific directions underwater and receiving echoes returned from underwater targets. There exist various types of sonar transducers, as illustrated in FIGS. 10 to 12, for example.
FIG. 10 is a perspective view of a cylindrical transducer 50 used in a full-circle scanning sonar. The transducer 50 includes a large number of transducer elements 51 arranged in horizontal rows and vertical columns on a cylindrical side surface of the transducer 50. The full-circle scanning sonar simultaneously transmits ultrasonic waves through the transducer elements 51 to form an acoustic sounding beam having an umbrellalike beam pattern directed obliquely downward at a specific tilt angle in all directions (360°) around the transducer 50 and receives echoes returned from a specific search area by scanning through the umbrellalike beam pattern. While the full-circle scanning sonar can sound a wide area not only horizontally but also vertically by electronically varying the tilt angle of the transducer 50, the full-circle scanning sonar has a disadvantage that it is impossible to sound straight down below a ship. This is because the sounding beam formed by the transducer elements 51 arranged on only the side surface of the transducer 50 can not be directed vertically downward.
FIG. 11 is a perspective view of a generally semicylindrical transducer 60 used in a half-circle scanning sonar. The transducer 60 includes a large number of elongate transducer elements 61 arranged side by side along a longitudinal direction of the transducer 60 on a generally semicylindrical side surface thereof which is formed as if by cutting part of a cylindrical shape. The half-circle scanning sonar simultaneously transmits ultrasonic waves through the transducer elements 61 to form an acoustic sounding beam having a fanlike beam pattern covering a half-circle (180°) area and receives echoes returned from a search area by scanning through the fanlike beam pattern. Designated by the numeral 62 is a training mechanism for horizontally rotating the transducer 60 in a direction marked by “H” in FIG. 11 by means of a motor (not shown) to direct the acoustic beam in a desired horizontal direction (bearing). Designated by the numeral 63 is a tilt mechanism for turning the transducer 60 in a direction marked by “V” in FIG. 11 by means of a motor (not shown) to adjust the direction (tilt angle) of the acoustic beam in a vertical plane. While the half-circle scanning sonar can sound across a wide area including a region straight down below a ship by mechanically varying the tilt angle and bearing of the transducer 60, the half-circle scanning sonar has a disadvantage that it is impossible to adjust vertical beam angle or sub-level of the acoustic beam by weighting in a direction perpendicular to the fanlike beam pattern. This is because each of the transducer elements 61 is not vertically divided into small segments.
It would be possible to overcome the aforementioned problem if both the cylindrical transducer 50 of FIG. 10 for full-circle scanning and the semicylindrical transducer 60 of FIG. 11 for half-circle scanning are installed on the hull of a ship. This approach is however impracticable from a viewpoint of cost and installation space, especially on a small ship. A scanning sonar employing a spherical transducer offering both full-circle and half-circle scanning features is made available for practical use today. For example, later-described Japanese Patent Application Publication Nos. 2000-162308 and 2001-343450 disclose examples of spherical transducers.
FIG. 12 is a perspective view of a spherical transducer 70 used in a scanning sonar. The transducer 70 includes a large number of elongate transducer elements 71 arranged on a spherical surface of the transducer 70. The scanning sonar transmits ultrasonic waves through the individual transducer elements 71 in specific directions and receives echoes by scanning through a search area. Since the transducer elements 71 are arranged on side and bottom areas of a spherical body of the transducer 70, the scanning sonar can sound across a wide area including a region straight down below a ship by using the single transducer 70 with a weight control capability. The transducer elements 71 of the spherical transducer 70 are not vertically arranged in straight lines like the cylindrical transducer 50 of FIG. 10, and directions in which radiating faces of the transducer elements 71 are oriented go further away from a horizontal plane toward the top and bottom of the spherical body of the transducer 70. For this reason, the spherical transducer 70 has a disadvantage that the intensity of a horizontal beam is rather low so that a large detection range is not achieved in horizontal directions and resolution in a vertical direction is relatively low.
One approach to aforementioned problems of the prior art would be to configure a transducer as a combination of a cylindrical portion and a hemispherical portion. Japanese Patent Application Publication No. 2001-343450 includes mention of a transducer having an upper cylindrical portion and a lower hemispherical portion. Taking advantage of the cylindrical transducer 50 of FIG. 10 and the spherical transducer 70 of FIG. 12, the transducer of this prior art Publication can sound a wide underwater area including a region straight down below a ship without sacrificing horizontal detection range or resolution in the vertical direction.
Manufacture of this kind of transducer having an upper cylindrical portion and a lower hemispherical portion requires some consideration of the arrangement of transducer elements. For example, it is necessary to minimize element-to-element interval in the vertical direction in order to suppress grating lobes which develop in the vertical direction during horizontal scanning. It is also necessary to give continuity to the transducer element arrangement on the upper cylindrical portion and the lower hemispherical portion in order to minimize deviation of directivity. There is no mention of such a transducer element arrangement in Japanese Patent Application Publication No. 2001-343450. On the other hand, Japanese Patent Application Publication No. 2000-162308 describes a configuration of a spherical transducer in which transducer elements are located at vertices of a regular polyhedron inscribed in a spherical surface as well as at midpoints of individual sides of each regular polygon constituting the regular polyhedron such that the transducer elements are located at substantially equal spacings from one another. According to the Publication, the transducer thus configured can minutely sound across an underwater search area in all directions at high speed with the transducer elements arranged uniformly and closely. While the Publication deals with the spherical transducer, it does not make any mention of an arrangement of transducer elements on a cylindrical portion of a transducer or continuity in the transducer element arrangement between the cylindrical and hemispherical portions. Thus, the aforementioned problems concerning the arrangement of transducer elements on the cylindrical and hemispherical portions for minimizing grating lobes and deviation of directivity still remain to be solved.